1. Field of the Invention
The present invention, in one application, relates to heat exchange and thermal insulation of apparatus for the electrochemical separation of oxygen from air or other oxygen rich gases for industrial, medical, and life support use, whereby heated, oxygen ion conducting ceramic devices are the means by which oxygen is separated from the oxygen containing gas mixtures. One type of electrochemical oxygen separator operates on the principle of high temperature electrochemical cells. The cells consist of separate electrode compartments and the electrodes are separated by, and in intimate contact with, predominantly oxygen ion conducting solid state electrolytes. Another type of electrochemical oxygen separator is based on oxygen ion conducting electrolytes which are also electronically conducting. This type of separator separates oxygen by means of establishing an oxygen pressure gradient across a mixed conducting (oxygen-ionic/electronic) oxide barrier, for instance, in the form of a tubular wall, to cause oxygen permeation through it and to emerge as pure oxygen gas on the other side of the barrier. Since both methods of oxygen separation are based on the oxygen ion conduction of pure ionic or mixed, oxygen-ionic/electronic, conductors, such separation devices will be called active electrochemical oxygen separation units.
An active electrochemical oxygen separation unit must be heated to temperature levels ranging between 500 and 900.degree. C. in order to increase the conductance of the oxygen ion conductors. As a consequence, oxygen containing feed gases must be heated to these temperature levels. The sensible heat of the oxygen depleted gases, exiting the active separation unit, must be recovered through heat exchange with the colder oxygen rich feed gases, in order to make electrochemical oxygen separators efficient in energy consumption, as well as acceptable, especially for uses such as patient home care and aeronautical life support systems, where excessive heating of closed environments must be avoided. For industrial applications energy efficiency is the most important reason for heat recovery.
The present invention relates, in another application, to the separation and/or purification of hydrogen from a gas mixture containing hydrogen. Microporous ceramic membranes are presently being developed which are permeable to hydrogen at elevated temperatures and under differential pressure conditions. Since hydrogen is a highly desirable energy commodity, such separation devices are of interest to many chemical and petrochemical process applications. In the following such hydrogen separation devices will be called hydrogen separators. The hydrogen separation process also requires efficient heat exchange between feed gases and hydrogen depleted exit gas in order to be efficient in energy consumption.
The application of this invention can be useful also in other gas separation processes, such as helium separation from helium containing gas mixtures by selective diffusion through membranes, which is similar to the hydrogen separation process, as well as in high temperature gas separation processes where heat exchange as well as thermal insulation is required.
2. Description of the Prior Art
Electrochemical devices that make rise of oxygen ion conductors for separation of oxygen from gas streams have been reported in the literature. U.S. Pat. No. Re. 28,792 (Ruka et al.) teaches the use of oxygen ion conducting electrolytes, such as yttria-stabilized zirconia in tubular form. When such a tube is coated on the inside and outside with platinuin electrodes, then heated in air while the electrodes are connected to a D.C. power supply, an ionic current of oxygen ions will now through the--otherwise gas impervious--tube wall from the cathode (negative terminal) to the anode (positive terminal). When the anode is made the inside electrode of the tube, pure oxygen will flow from the tube inside.
Oxygen ion conducting electrochemical devices have been constructed with solid electrolytes such as doped bismuth oxide and doped cerium oxide, as reported in the literature by E. N. Naumovich et al., Solid State Ionics 93, 1997, 95-103, and H. Inaba, H. Tagawa, Solid State Ionics 83, 1996, 1-16, respectively. Such oxide mixtures are investigated because they have a higher oxygen ion conductivity than stabilized zirconia
U.S. Pat. No. 5,045,169 (Feduska et al.) teaches an electrochemical oxygen separator which makes use of a thin layer of solid electrolyte made of stabilized zirconia. The described oxygen separator consists of a multiple cell stack where cells are electrically connected in series. The cell stack is supported on a porous stabilized zirconia tube, which, in addition, serves as oxygen delivery conduit. The same electrochemical oxygen separator cell stack structure is applied in U.S. Pat. No. 5,186,793 (G. A. Michaels), it teaches a high temperature electrochemical oxygen generator whereby some recuperative heating of feed air is accomplished, however, the sensible heat of the partially oxygen depleted air is dispersed through mixing with large amounts of ambient air.
An example of heat exchange with respect to a high temperature electrochemical oxygen separator is taught by U.S. Pat. No. 5,332,483 (A. Z. Gordon), wherein air is preheated in a heat exchanger with spent air from a bipolar electrolysis module, the heat exchanger and electrolysis module being separate units connected via gas ducts.
An oxygen separator which is based on mixed conducting electrolyte membranes through the application of differential oxygen pressures is disclosed in EPRI Final Report GS/ER-7097, Res. Proj. 1676-11, 8002-16, (Feduska et al.), which describes the mode of operation and composition of a mixed conducting oxygen ion conductor on the basis of zirconia. Another publication in Solid State Ionics 72 (1994) pp 185-194 (H. J. M. Bouwmeester et a).) describes the use of perovskite type mixed conducting oxide materials for electrochemical oxygen separator membranes. A large number of mixed oxides in the family of perovskites are potential candidates for device applications, such as SrFe.sub.1-x Co.sub.x O.sub.y, LaNi.sub.1-x Co.sub.x O.sub.3, and many others.
At this point in time, there are no known electrochemical oxygen separators for the intended applications such as home patient care (which need to deliver 3 to 5 liters of oxygen per minute) commercially available. Specific information, therefore, on prior art with respect to efficient heat exchange and thermal insulation for high temperature electrochemical oxygen separators is scarce.
General information on heat exchange of gas processes is plentiful and a large number of heat exchanger construction principles and materials of construction are available. The following literature teaches most aspects of heat transfer: Compact Heat Exchangers, W. M. Kays, A. L. London, The Natl. Press, Palo Alto, Calif. 1955; Heat Exchangers: Design and Theory Source Book, N. H. Afgan, E. V. Schlunder, Edts., McGraw-Hill Book Co., 1974; Handbook of Heat Transfer, W. M. Rohsenow, J. P. Hartnett Process Heat Transfer, D. Q. Kern, McGraw-Hill Book Co. Inc., 1950; Principles of Heat Transfer, 3rd Edition, F. Kreith, Harper & Row, Publishers, 1973.
A heat exchanger principle, relevant to this invention has been published in Ullman, 1, p. 226, Urban & Schwarzenberg, Publishers, 1951, it describes a heat exchanger exhibiting two separate and sealed spiral flow channels as well as two annular as well as two peripheral gas ports.
In general, most heat exchanger concepts, whether based on the tube and shell or plate type, are built with absolutely leak free flow channels in mind for the separate gas streams which are undergoing heat exchange. Furthermore, heat exchanger technology, traditionally, strives for structural integrity and mechanical stability because the heat transfer devices are often used as pressure vessels or as multiple vessels combined in one pressure envelope. Heat exchangers, therefore, are constructed specifically for various applications, and electrochemical oxygen separators are a typical example of such a case for which a suitable heat exchange concept is needed.
Conventional as well as compact advanced heat exchangers, such as tube and shell or plate type, are much to heavy for portable electrochemical oxygen separators. The maximum practical temperature level for oxygen separators (900.degree. C.) presents another materials limitation that severely limits the availability of useful heat exchanger technology for high temperature oxygen separators especially when pressurized operation of oxygen separators is considered. The strength of heat exchanger vessel alloys is severely reduced at elevated temperatures and pressure operation would, at best, be possible only through increased vessel wall thickness.
Similar problems exist in applying advanced insulation technology to high temperature oxygen separators. There is a number of efficient fibrous ceramic insulation materials commercially available, such as FIBER, SAFFIL AND SAFFIL-derived products which meet temperature stability and insulation criteria, however, all these products are voluminous and have a weight to volume ratio that is too high for portable oxygen generators.
As a result of these considerations one must come to the conclusion that state-of-the-art heat exchanger and insulation technology cannot be applied to portable electrochemical oxygen separators.
In another application of the invention, the separation of hydrogen from mixtures with other gases has been investigated for many years and commercial processes have been developed. The best developed process is that of selective diffusion of hydrogen through palladium silver alloy membranes at 300 to 400.degree. C. U.S. Pat. Nos. 2,911,057 (Engelhard In.); 2,961,062 (Atl. Ref.Co.) teach diffusion separation of hydrogen in palladium/silver diffusion cells. Small laboratory size units of the palladium type are electrically heated without heat recovery, larger units are recuperatively heated. Palladium based hydrogen separators are expensive and are easily irreversibly poisoned by gas impurities such as sulfur compounds. These are the major reasons for the development of molecular sieve based microporous separators which are inexpensive. State-of-the-art palladium membrane technology does not offer advanced low cost and highly efficient heat exchanger and insulation technology which would be helpful in bringing modern highly efficient hydrogen gas separators to a successful commercial application.
Similar considerations are valid for the separation of helium by diffusion from industrial as well as geological gas streams such as natural gas. A French Patent No. 698,822, 1930, teaches the purification of helium by diffusion through quartz at high temperatures. State-of-the-art helium separation, however, is conducted by energy intensive cryogenic methods. Helium separators using micropoous molecular sieve type membranes, which operate at elevated temperatures and differential pressures, offer significant savings in energy consumption.
Ceramic membranes for high temperature hydrogen and helium separation are the subject of investigation in a publication of the Proc. of the 10th Annual Conf. on Fossil Energy Materials, May 1996, Knoxville, Tenn., p. 107 (D. E. Fain, G. E. Roettger).